Skip to main content

Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

See other formats

442                                 ALASKAN GLACIER STUDIES
margin. A medial moraine near the south side is broad and on passing outside the mountain valley it swings across into the middle of the glacier with a shape like a fishhook. At the mouth of the mountain valley the glacier grade is steepened slightly and the ice is more crevassed than in the piedmont bulb. This may be interpreted as evidence that Allen Glacier is in a hanging valley.
The Allen Glacier Bulb. While within its mountain valley the glacier has a width of only a mile and three quarters to two miles, it expands in the broad Copper River valley forming a bulb five miles wide (PL CLXXIV), with a grade which flattens to 180 feet to the mile. The medial moraine and the southern lateral* spreading out in this piedmont bulb, cover the southern part of the glacier with a veneer of moraine; but the northern part of the bulb, for a width of a mile and a half consists of almost-uncrevassed ice with practically no moraine covering. Here a few ice cones, with protecting moraine veneer, stand above the general level, as they also do near the center of the glacier at the margin of the clear ice area. The northern margin of the bulb has several medial moraines and a hummocky lateral moraine. These are all made up of black shale, greenstone, and graywacke, except one of yellow granite, and their prevailing dark color forms a striking contrast with the red moraines of the southern border. On the north much less of the margin of the bulb is moraine-covered. On each margin of the bulb close to the debouchure of the valley glacier there is a narrow barren zone of naked rock.
The Terminal Hill. East of this clear ice area, and rising to a height of a little less than 500 feet, is a wooded hill zone, which extends from the zone of barren ablation moraine about a mile eastward to the river. This terminal hill is the extension of the glacier bulb; although it does not look like a glacier, it is everywhere underlain by ice. Its highest part is nearest the clear ice, from which it is separated by an abrupt crescentic escarpment facing the glacier. The undulating eastern slope is a long gentle descent to the edge of the marginal channel of Copper River in Baird Canyon. This surface is rather hummocky and is slumping here and there, so that sections exposing bare ice, beneath ablation moraine and beneath a tangle of roots, are frequently revealed. Ice was seen in several places along the river bank in August, 1909, and many other exposures were seen when the railway was being constructed.
There are valleys cut in the moraine-covered glacier margin all along its outer periphery, in one case with a considerable basin holding a lake and doubtless due more to melting and slumping than to stream cutting.
Upon this stagnant, moraine-covered outer portion of the bulb grows a dense, continuous, mature, alder thicket* with scattered cottonwoods near the river. The annual rings in several of the largest alders were counted in 1910 and the ages varied from 50 to 67 years, indicating that the advance of the glacier across the valley, which we know from Allen's observations to have been over 25 years ago was certainly over 67 years ago (before 1843) and perhaps even earlier.
The Interior Flat. The clear ice area within the vegetation-covered bulb is an interior flat (PI. GLXXII), intermediate in stage of development between that of Atrevida Glacier in 1909 and those of Variegated and Galiano Glaciers in 1905-09. In August, 1909, this interior flat was bounded by the crescentic escarpment of the forested terminal hill and by a terminal moraine. It represented the site of 200 feet or more of melting of clean ice, lowering below the former level, as indicated by the forested part of the ice